Abstract The long-term objectives of this application are to understand the molecular mechanisms which control breast cancer bone metastasis. Breast cancer frequently metastasizes to bone in patients with advanced diseases. In contrast to prostate cancer which forms osteoblastic lesion, bone metastasis of breast cancer typically leads to osteolysis which is often accompanied with severe pain, pathological fracture, and hypercalcaemia. Although the molecular mechanism underlying the preferential bone metastasis is yet to be elucidated, it is believed that osteoclasts activated by breast cancer cells, but not breast cancer cells themselves, mediate osteolysis. Osteoclasts play an initiating and pivotal role in stimulating bone-metastatic tumor growth in the marrow cavity. While bone microenvironments allow circulating breast cancer cells to preferentially survive and grow, bone-seeking breast cancer cells may have intrinsic properties to promote osteoclast formation and activation. The central hypothesis of this application is that constitutive nuclear factor-kappa B (NF-B) activities in breast cancer cells play an integral role in osteolytic bone metastasis and osteoclastogenesis. NF-B is an inflammatory transcription factor which regulates a broad range of gene expression, including pro-inflammatory cytokines, chemokines and cell adhesion molecules. Using in vitro and in vivo model systems, we have found that constitutive NF-B activities in breast cancer cells promote osteolytic tumor growth by stimulating osteoclastogenesis. In this application, based on our exciting preliminary studies, we will explore how NF-B is disregulated and recruited to chromatin to stimulate target gene transcription in breast cancer cells using a combination of biochemical and genetic approaches. We will determine whether NF-B-induced genes such as chemokines secreted by breast cancer cells stimulate osteoclast formation, thereby promoting osteolytic bone metastasis. Intriguingly, NF-B is also essential for osteoclastogenesis, and pro-inflammatory cytokines and chemokines have been found to promote osteoclast differentiation by activating NF-B. However, unlike cytokines, little is known about how chemokines activate NF-B. Therefore, we will also explore how chemokines trigger an intracellular signaling cascade to activate NF-B to promote osteoclast differentiation. Taken together, the results from this application will provide novel insights into osteolytic bone metastasis and osteoclast activation by NF-B, and help to develop new strategies for treating skeletal bone metastasis.